netkit: Set IFLA_NETKIT_PEER_INFO to netkit_link_ops.peer_type.

For per-netns RTNL, we need to prefetch the peer device's netns.

Let's set rtnl_link_ops.peer_type and accordingly remove duplicated
validation in ->newlink().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20241108004823.29419-9-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>